74 Duster 318 with 904 Trans

I purchased a 74 Duster that has been converted from a I6 to a 318. The engine runs great and smooth when not in gear. The engine will die when placed in gear unless the idle is set high. The car runs and drives fine other than the need to be at high idle to keep running. It is a stock engine with an Edelbrock 4 barrel carb and aftermarket intake with headers. I am not sure if this is a carb tuning issues or maybe a torque converter issue. Any ideas or thoughts on where I should start to address this issue?